Typically, a vehicle is equipped with an occupant protection system for protecting an occupant from injury in a collision. As examples of the occupant protection system, an airbag system inflates an airbag to protect a head of the occupant and a pretensioner system takes up slack in a seatbelt to firmly hold the occupant in the collision. Such an occupant protection system is controlled by an electrical control unit (ECU). The ECU determines whether the collision occurs based on a sensor signal from a sensor mounted on the vehicle. When the ECU determines that the collision occurs, the ECU activates the occupant protection system.
Recently, there has been an increase in demand for not only front/rear impact safety but also side impact safety. A side airbag system, disclosed in, for example, JP-H2-249740A, protects the occupant from a side impact in a side collision. A pressure sensor used in the side airbag system measures a change in pressure inside a door of the vehicle to detect the side impact.
FIG. 3 shows a conventional pressure sensor 1 for detecting the side impact. The conventional pressure sensor 1 has a hollow case 2 with an air passageway 22 through which air is introduced into the case 2. A circuit assembly 3 housed in the hollow case 2 includes a circuit board 30 and a pressure sensing element 31 mounted on the circuit board 30. The conventional pressure sensor 1 needs to be waterproofed. Therefore, a rubber gasket (packing) member 6 is placed between the case 2 and the pressure sensing element 31 to prevent water from entering the conventional pressure sensor 1 except for a surface of the pressure sensing element 31. Thus, the circuit board 30 is protected from water damage so that the circuit board 30 can work properly.
Each of the case 2, the circuit board 30, and the pressure sensing element 31 requires a high dimensional accuracy to seal the inside the conventional pressure sensor 1 with the rubber gasket member 6. This requirement increases manufacturing cost of the conventional pressure sensor 1.
The rubber gasket member 6 is pressed between the case 2 and the pressure sensing element 31 mounted on the circuit board 30 and stress is applied to an electrical junction between the circuit board 30 and the pressure sensing element 31 by the case 2. Therefore, it is difficult to ensure reliability of the electrical junction, circuit elements mounted on the circuit board 30, and the pressure sensing element 31.
The conventional pressure sensor 1 further includes a potting member 25 for supporting the circuit assembly 3. Whereas the potting member 25 improves waterproofness of the conventional pressure sensor 1 and reduces stress concentration on the circuit board 30 and the pressure sensing element 31, the potting member 25 increases the manufacturing cost of the conventional pressure sensor 1.